Triple negative, basal-like breast cancer is highly aggressive and has a poor prognosis. The molecular mechanisms that drive its progression are not well understood and no molecular target has been identified for its prevention or treatment. Recent gene profiling studies showed that immortalized human mammary epithelial cells (HMECs) resemble basal-like breast cancer and are appropriate models for the study of basal-like breast cancer. Thus, our goal is to identify molecular mechanisms that cause basal-like breast carcinogenesis using HMECs. We found that abrogation of autocrine TGF? signaling with the expression of a dominant-negative Rll (DNRII) in telomerase-immortalized HMECs suppressed H-ras-V12-induced senescence-like growth arrest (SLGA). Furthermore, co-expression of DNRII and H-ras-V12 rendered HMECs to become highly tumorigenic and metastatic in vivo whereas HMEC clones that spontaneously escaped H-ras-V12-induced SLGA were only weekly tumorigenic and non-metastatic. Thus, we have generated a series of isogenic HMECs representing a full spectrum of basal-like breast carcinogenesis without directly inactivating p53. To identify candidate genes that may mediate the escape of H-ras-V12- induced SLGA and drive malignant progression, we have determined gene expression profiles of five HMEC lines with different senescent and malignant properties. Among the alterations of many interesting genes are down-regulated p21cip1/waf1 and a group of up-regulated genes in DNRII and DNRII+ras HMECs, whose products are involved the regulation of cell cycle checkpoints and transition through mitosis. Thus we hypothesize that autocrine TGF? signaling is an integral part of cellular anti-transformation network by suppressing the expression of a host of genes that mediate oncogene-induced transformation, and abrogation of autocrine TGF? signaling can promote basal-like breast cancer progression. Utilizing the isogenic HMECs we have generated, we will determine the role of autocrine TGF? signaling in promoting oncogene-induced SLGA, chromatin remodeling, and the regulation of p53 activity. We will also determine how the interactions between TGF?, p21, and p53 pathways may confer the role of TGF? in breast carcinogenesis with in vivo approaches. Our long-term goals are to identify biomarkers for basal-like breast cancer prognosis and novel therapeutics to inhibit basal-like breast cancer metastasis.